Nitrate (NO3 - -N) accumulation in soils resulting from excessive nitrogen (N) fertilization increases risks of N loss to the environment. Microbial NO3 - -N immobilization (INO3) can be stimulated by adding high-organic carbon (C) materials to soils and has potential to control N loss from agricultural soils during periods of NO3 - -N accumulation. It is therefore important to screen for such organic materials that show a rapid and high capacity of INO3. The C/N ratio of organic materials is considered a primary factor controlling soil INO3, but this is challenged for high-organic C soil amendments that have similar C/N ratios but contain components with very different microbial degradability. Our objective was to identify better predictors of the INO3 potential of high organic C materials with comparable C/N ratios. Using a 15N-tracing laboratory experiment on one nitrate-rich agricultural soil with and without 11 types of organic C materials, we observed that those with similar C/N ratios (>18) increased the INO3 to varying degrees. Once organic amendments exceed a C/N ratio of 18, this ratio may no longer be an ideal index for estimating rapid INO3 potential. We provide evidence that the C mineralization of high-organic C soil amendments, determined as CO2 emission over 12 days, more accurately predicts the potential for rapid INO3 in soil.